This invention relates to a chain tensioner for keeping a constant tension of a chain such as a camshaft driving chain in an internal combustion engine.
Typically, a chain transmission system for driving camshafts of a vehicle engine includes a chain tensioner for applying pressure to the slack side of the camshaft driving chain to keep a constant tension in the chain.
A typical conventional chain tensioner comprises a housing formed with a cylinder chamber, a plunger slidably received in the cylinder chamber, and a spring mounted in the cylinder chamber and biasing the plunger out of the cylinder chamber. The plunger defines a damper chamber in the housing. The housing is formed with an oil supply passage through which hydraulic oil is supplied into the damper chamber. A check valve is provided at the outlet of the oil supply passage. Hydraulic oil supplied into the damper chamber through the oil supply passage dampens the force applied to the plunger.
When the engine is stopped, according to the positions of the cams when the engine is stopped, the chain may remain in a tensioned state. The chain will thus push the plunger rather deeply into the cylinder chamber after the engine has stopped. When the engine is restarted with the plunger deeply pushed into the cylinder chamber, the plunger will advance abruptly for a rather long distance. However, because a hydraulic pump for supplying hydraulic oil into the damper chamber through the oil supply passage has just been started, it cannot fill the damper chamber, of which the volume is quickly increasing, with oil at a sufficient speed. This may cause the inclusion of air in the damper chamber. Air in the damper chamber will greatly deteriorate the function of the chain tensioner as a damper. Noise may result, too.
The chain tensioner disclosed in JP patent publication 2001-355691A includes means for obviating this problem. Specifically, as shown in FIGS. 11 to 14, this chain tensioner includes a housing 71 defining a cylinder chamber 72 formed with a ring-receiving groove 73 in its inner periphery near its open end. A radially and resiliently deformable register ring 74 is received in the ring-receiving groove 73. A plunger 76 is slidably mounted in the cylinder chamber 72 and is biased outwardly by a spring 75. In the outer cylindrical surface of the plunger 76, a plurality of annular grooves 77 are formed so as to be axially spaced from each other at equal intervals. Each annular groove 77 comprises a rear tapered surface 77a of which the diameter decreases gradually toward the front end of the plunger, and a front engaging surface 77b extending from the front small-diameter end of the tapered surface toward the front end of the plunger. The ring-receiving groove 73 includes a locking surface 73a at its rear portion. The engaging surface 77b of any of the annular grooves 77 and the locking surface 73a are capable of trapping the register ring 74 therebetween, thereby preventing retraction of the plunger 76.
Further, in order that this chain tensioner can be mounted easily, the plunger 76 has a small-diameter portion 78 at its front end, thereby defining a set surface 79 at the boundary between the small-diameter portion 78 and the rear large-diameter outer cylindrical surface 78a. By trapping the register ring 74 between the set surface 79 and a stop surface 73b formed at the front portion of the ring-receiving groove 73, the plunger 76 can be kept pushed in the cylinder chamber.
Once this chain tensioner is mounted, the register ring 74 is radially expanded to unlock the plunger 76. The plunger 76 is thus pushed outwardly by the spring 75, thereby pressing a pivotable chain guide 80 against a chain 81. The chain 81 is thus kept in a tensioned state.
If the chain guide 80 is simply pushed by the plunger 76, the chain guide 80 may move laterally due to vibration of the chain. In order to check such lateral movement of the chain guide, the chain guide 80 is formed with a guide groove 82 in which the small-diameter portion 78 of the plunger 76 is received.
When the register ring 74 is radially expanded to disengage it from the set surface 79, the plunger 76 will move outwardly under the force of the spring 75, so that as shown in FIG. 12, the register ring 74 will engage in the frontmost one of the annular grooves 77 (which is hereinafter called the first annular groove). In this state, due to changes in the tension of the chain resulting from fluctuations in the torque of the camshafts, the chain 81 vibrates, thereby causing the plunger 76 to repeatedly move back and forth as shown in FIGS. 11 to 13.
In this arrangement, if the outer cylindrical surface 78a extending from the outer (rear) edge 79a of the set surface 79 to the first annular groove 77 is short in axial length, while the plunger 76 is moving back and forth, the outer cylindrical surface 78a may separate from the inner cylindrical surface 72a of the cylinder chamber 72 extending forwardly from the ring-receiving groove 73, as shown in FIGS. 13 and 14. When the plunger 76 is pushed into the cylinder chamber 72 in this state, the plunger 76 tends to incline, which will promote wear of the inner peripheral surface of the cylinder chamber 72 and the outer cylindrical surface 76a of the plunger 76 and may also impair operation of the register ring 74.
Also, when the plunger 76 advances under the force of the spring 75, the edge 79a of the set surface 79 will abut the inner (front) edge 73b′ of the stop surface 73b of the ring-receiving groove 73, impairing smooth movement of the plunger 76.
Among chain tensioners having means for restricting retraction of the plunger 76, there are ones in which the set surface 79 shown in FIG. 12 is omitted, and instead, the plunger is formed with a small-diameter portion at the front end thereof for preventing lateral movement of the chain guide 80, the small-diameter portion connecting with the rear large-diameter portion of the plunger through an inclined surface. In such chain tensioners, too, if the cylindrical surface of the plunger 76 extending between the outer edge of the inclined surface and the first annular groove is short, the same problems as mentioned above may arise.
In any of the abovementioned conventional chain tensioners, since the plunger 76 is always biased outwardly of the cylinder chamber by the spring 75, when mounting such a chain tensioner, it is necessary to fix the housing 71 to an engine block with the plunger 76 kept pushed into the housing. Thus, it was extremely troublesome to mount conventional chain tensioners to engine blocks.
A chain tensioner has been proposed which is free of this problem. That is, this tensioner is formed with radial pin holes in the front end of the plunger and the front end of the housing. With the plunger pushed into the housing until the pin hole formed in the plunger aligns with the pin holes formed in the housing, a set pin is inserted through the pin holes to keep the plunger pushed in the housing. After the chain tensioner has been mounted on an engine block, the pin is removed to let the plunger advance under the force of the spring, thereby applying tension to the chain.
But if an operator forgets to remove the set pin after mounting the chain tensioner on the engine block, the plunger is kept retracted in the housing. The chain tensioner therefore cannot perform its expected function.
Since an engine cover can be mounted on the engine block thereafter even if the set pin remains in the chain tensioner, once an operator forgets to remove the pin, he may leave the pin in the chain tensioner, not knowing this fact.
A first object of the present invention is to provide a chain tensioner which can minimize the wear of the sliding surfaces of the cylinder chamber and the plunger, and ensures smooth sliding movement of the plunger and unhindered operation of the register ring.
A second object of the invention is to provide a chain tensioner including means for preventing an operator from forgetting to remove the set pin after the chain tensioner has been mounted.